1. Field of the Invention
This invention relates generally to the field of hydrogenation. More specifically, the invention relates to methods and catalyst for hydrogenation of unsaturated fatty acid compositions to yield triglyceride compositions having reduced levels of trans fats. More particularly, the present invention relates to a catalyst for the activation of fatty acids and a high shear process for improving the hydrogenation reaction. The disclosed process creates conditions of temperature, pressure and contact time such that hydrogenation may be accelerated beyond what is traditionally encountered in industry.
2. Background of the Invention
Chemical reactions involving liquids, gases and solids rely on the laws of kinetics that involve time, temperature, and pressure to define the rate of reactions. In cases where it is desirable to react two raw materials of different phases (i.e. solid and liquid; liquid and gas; solid, liquid and gas), one of the limiting factors in controlling the rate of reaction involves the contact time of the reactants. In the case of catalyzed reactions there is the additional rate limiting factor of having the reacted products removed from the surface of the catalyst to enable the catalyst to catalyze further reactants.
From a chemical perspective, fats are large molecules that support three fatty acid groups connected to a short backbone derived from glycerol, superficially resembling an E. What is commonly termed a trans fat is more accurately described as a fat that contains a trans fatty acid group. Fatty acid molecules consist of a backbone of carbon atoms, each with attached hydrogen atoms (as well as a carboxyl group positioned at the end of the molecule, which is not pertinent to this discussion). Fatty acids are characterized as saturated or unsaturated based on the number of double bonds in the acid. If the molecule contains the maximum possible number of hydrogen atoms then it is saturated; otherwise, it is unsaturated.
Carbon atoms are tetravalent, forming four covalent bonds with other atoms, while hydrogen atoms bond with only one other atom. In saturated fatty acids, each carbon atom is connected to its two neighboring carbon atoms as well as two hydrogen atoms. In unsaturated fatty acids the carbon atoms that are missing a hydrogen atom are joined by double bonds rather than single bonds so that each carbon atom participates in four bonds.
Hydrogenation of unsaturated carbon to carbon double bonds is commonly practiced in petroleum and chemical processing operations as well as in vegetable based oils processing. The main purpose of hydrogenation is to increase the stability of the oil and/or alter its physical properties. Although the focus of this invention is mainly on hydrogenation of fatty acids, the process can readily be applied to any unsaturated liquid hydrocarbon including petroleum products.
Hydrogenation of an unsaturated fatty acid refers to the addition of hydrogen atoms to the acid, converting double bonds to single bonds as carbon atoms acquire new hydrogen partners (to maintain four bonds per carbon atom). Full hydrogenation results in a molecule containing the maximum amount of hydrogen (in other words the conversion of an unsaturated fatty acid into a saturated one). Partial hydrogenation results in the addition of hydrogen atoms at some of the empty positions, with a corresponding reduction in the number of double bonds. Commercial hydrogenation is typically partial in order to increase stability and/or to obtain a malleable fat that is solid at room temperature, but melts upon baking (or consumption).
Oils extracted from vegetable seeds, and from produce such as soy, corn, rapeseed and the like consist primarily of triglycerides, a glycerin molecule combined with three fatty acid molecules. Vegetable oils derived from different sources differ from each other in the fatty acid component of the triglycerides. Fatty acids vary in both the length of carbon chain, and the number of double bonds present in those carbon chains. The majority of fatty acids in vegetable oils have carbon chain lengths varying from about C8 to about C20.
Hydrogenated vegetable oils are generally produced by contacting hydrogen gas with vegetable oil in the presence of a catalyst. Hydrogenation is used, for example, to increase the chemical stability of triglycerides comprising the oil, and/or to increase the triglyceride content that is solid at room temperature, as the hydrogen reacts with carbon-carbon double bonds of the fatty acid moieties of the triglycerides.
Triglyceride-based vegetable fats and oils can be transformed through partial or complete hydrogenation into fats and oils of higher melting point. The hydrogenation process typically involves “sparging” the oil at high temperature and pressure with hydrogen in the presence of a catalyst, typically a powdered nickel compound. As each double-bond in the triglyceride is broken, two hydrogen atoms form single bonds. The elimination of double-bonds by adding hydrogen atoms is called saturation; as the degree of saturation increases, the oil progresses towards being fully hydrogenated. An oil may be hydrogenated to increase resistance to rancidity (oxidation) or to change its physical characteristics. As the degree of saturation increases, the viscosity and physical state of the oil may be changed (liquid to solid).
The use of hydrogenated oils in foods has never been completely satisfactory. Because the center arm of the triglyceride is shielded somewhat by the end triglycerides, most of the hydrogenation occurs on the end triglycerides. This makes the resulting fat more brittle. A margarine made from naturally more saturated tropical oils will be more plastic (more “spreadable”) than a margarine made from hydrogenated soy oil. In addition, partial hydrogenation can result in the formation of trans fats, which have, since about the 1970s, been increasingly viewed as unhealthy. In conventional hydrogenated vegetable oils, the hydrogenation process converts many of the double bonds from the cis position to the trans position. These trans fatty acids are undesirable for human consumption due to the association of trans fatty acids with adverse health effects, such as hypercholesterolemia.
Because of current health concerns about the levels of trans fats in foods, it is desirable to produce edible fats and oils that can be labeled as containing “zero trans fat”. Current regulations issued by the U.S. Food and Drug Administration, effective Jan. 1, 2006 allow for products with trans fat levels of less than 0.5 grams per serving to be labeled as containing ‘zero trans fat’ (68 Federal Register 41434 (2003)). As used herein the term “low trans fat” will refer to levels of trans fat that would qualify products containing them to be labeled as “zero trans fat” in accordance with these regulations.
Low trans fat products (e.g., certain margarines and hydrogenated vegetable oils) are generally formed from a blend of inter-esterified fats, unsaturated vegetable oils, saturated vegetable oils and mixtures thereof. While these processes produce a low trans fat product, the product is often high in saturated fats. Saturated fats are also not desirable for human consumption due to adverse health effects. Other methods of reducing trans fat while trying to minimize the formation of saturated fat have been disclosed, but none have proven satisfactory. For example, one can lower the stearic acid (C18:0) content of a hydrogenated oil by chilling the oil and solidifying the saturated fat, followed by physical separation, as known to those skilled in the art (Food Industries Manual, 24th Edition, 1997, Christopher G J Baker; Published by Springer; pp. 289-291).
There are numerous patents concerning hydrogenation of triglyceride to control the levels of trans fat or the level of saturated fats.
U.S. Pat. No. 5,064,670 (Hirshorn et al.) describes a frying fat exhibiting a reduced concentration of saturates and a method of frying food products as well as frying confectionaries such as doughnuts. Such fat products are produced by blending various oils to the desired properties for frying or confectionary use.
U.S. Pat. No. 5,194,281 to Johnston et al. describes polyol fatty acid polyesters with reduced trans double bond levels and a process for making them.
United States Pat. App. Pub. No. 2005/0027136 A1 (Van Toor et al.) describes a process to hydrogenate vegetable oils with an activated catalyst. The process uses pressures ranging from about 7 to about 30 bar (from about 100 psi to over 400 psi), and reaction times ranging from 100 minutes (“min”) to over 400 min. Van Toor et al. note that such long hydrogenation times (460 min) may prove unduly expensive for low cost frying oils, margarines, bakery fats or similar applications. Most commercial equipment used for hydrogenation utilizes pressures in the range of 60 psi, and with reaction times of 60 to 90 minutes. The Iodine Values of the hydrogenated oils produced in the examples are not provided, such that it is difficult to determine the extent of hydrogenation.
With rare exception, no reaction below 480° C. occurs between H2 and organic compounds in the absence of metal catalysts. The catalyst simultaneously binds both the H2 and the unsaturated substrate and facilitates their union. Platinum group metals, particularly platinum, palladium, rhodium and ruthenium, are highly active catalysts. Highly active catalysts operate at lower temperatures and lower pressures of H2. Non-precious metal catalysts, especially those based on nickel (such as Raney nickel and Urushibara nickel) have also been developed as economical alternatives but they are often slower and/or require higher temperatures. The trade-off is activity (speed of reaction) vs. cost of the catalyst and cost of the apparatus required for use of high pressures.
Two broad families of catalysts are known; homogeneous catalysts and heterogeneous catalysts. Homogeneous catalysts dissolve in the solvent that contains the unsaturated substrate. Heterogeneous catalysts are solids that are suspended in the same solvent with the substrate or are treated with gaseous substrate. In the pharmaceutical industry and for special chemical applications, soluble “homogeneous” catalyst are sometimes employed, such as the rhodium-based compound known as Wilkinson's catalyst, or the iridium-based Crabtree's catalyst.
The activity and selectivity of catalysts can be adjusted by changing the environment around the metal, i.e. the coordination sphere. Different faces of a crystalline heterogeneous catalyst display distinct activities, for example. Similarly, heterogeneous catalysts are affected by their supports, i.e. the material upon with the heterogeneous catalyst is bound. Homogeneous catalysts are affected by their ligands. In many cases, highly empirical modifications involve selective “poisons.” Thus, a carefully chosen catalyst can be used to hydrogenate some functional groups without affecting others, such as the hydrogenation of alkenes without touching aromatic rings, or the selective hydrogenation of alkynes to alkenes using Lindlar's catalyst. For prochiral substrates, the selectivity of the catalyst can be adjusted such that one enantiomeric product is produced.
Unsaturated triglycerides are refractory towards hydrogenation and typically require high temperature, high pressure, protracted hydrogenation time or combinations thereof in order to obtain satisfactory hydrogenation. Conventionally, unsaturated triglycerides are hydrogenated with hydrogen gas in the presence of at least 0.2 to 0.5% nickel hydrogenation catalyst and occasionally more at temperatures around or above 150° C. under pressures of from 60 psig to 100 psig and higher. Times of at least 1 to 8 hours or more are required depending on the degree of hydrogenation desired. By contrast, hydrogenation of glyceride oils (which generally are not refractory towards hydrogenation) typically can be accomplished in relatively short times at about 100° C.-260° C. at pressures of around 0 psig to 100 psig. Fatty acids, then, are adjudged to be refractory towards hydrogenation by comparison and contrast to glyceride oils. Hydrogenation of fatty acids and glyceride oils is outlined in Bailey's Industrial Oil and Fatty Products, 3rd Edition, pp. 719-896 (Interscience Publishers, New York, N.Y., 1964), the same being expressly incorporated herein by reference. A continuous process for the hydrogenation of fatty acids is also described in U.S. Pat. Nos. 5,382,717 and 4,847,016, which are hereby incorporated herein for all purposes.
As can be seen in the discussion above, technology involving the hydrogenation of fatty acids has focused on improving the catalysts required for hydrogenation. To this point, methods of improving mass transfer of hydrogen into unsaturated fatty acids or lowering the temperature of the hydrogenation reaction have not heretofore been addressed.
Numerous devices have been proposed for accelerating the rates of reaction for reactions other than the hydrogenation of fatty acids. For example, there has been disclosure by Shah et al. (Cavitation Reaction Engineering, ISBN 06461412) of a method of accelerating chemical reactions through the use of hydrodynamic cavitation. Hydrodynamic cavitation occurs when the pressure variation caused by the variation in the flowing liquid velocity results in a phase change and rapid increases in temperatures and pressures that result in accelerated chemical reaction.
In conventional reactors, contact time for the reactants and or catalyst is often controlled by mixing which provides contact with two or more reactants involved in a chemical reaction. There have been various innovations directed towards maximizing the use of mixing and mixing devices to accelerate chemical reactions.
High shear and high energy mixers are well known devices that have been reported for use in some chemical reactions. For example, U.S. Pat. No. 7,138,434 (Huff et al.) describes a process for converting synthesis gas to higher hydrocarbons by introducing a synthesis gas feed stream into a continuous stirred reactor system comprising a reactor vessel containing a suspension of a solid particulate Fischer-Tropsch catalyst suspended in a liquid medium.
U.S. Pat. No. 6,822,007 (Ketley et al.) describes a process for converting synthesis gas into higher hydrocarbons utilizing a high shear mixing zone and a tubular loop reactor where the high shear mixing zone is an injector-mixing nozzle.
U.S. Pat. No. 6,502,980 (Ekstrom et al.) discloses the use of in-line homogenizer using rotors and stators in a housing for creating emulsions, suspensions and blends used in pharmaceutical, biological, cosmetic, chemical and food compositions.
United States Patent Application No. 20050130838 (Duan, Xue, et al.) discloses the use of a colloid mill reactor to produce a nano-scale magnetic solid base catalyst.
U.S. Pat. No. 5,369,167 (Pottick, et al.) describes a process for melt blending acid or anhydride-grafted block copolymer pellets with epoxy resin. The epoxy resin-modified block copolymer blend is held under high shear mixing under conditions sufficient to react an amount of the modified hydrogenated block copolymer functional groups with epoxy groups effective to provide a stable dispersion of the modified hydrogenated block copolymer in the epoxy resin.
The term ‘high-shear mixer’ has been used to describe non mechanical mixers. U.S. Pat. No. 6,235,961 (Kurukchi) describes a process for pretreating cracked gas before a caustic tower treatment in ethylene plants which effectively increases the efficiency and capacity of the caustic tower by using a high-shear mixer, such as an inline, cocurrent-flow static mixer or a venturi scrubber with a caustic solution and the cracked gas.
In recognition of the need to provide contact between reactants in chemical reactions, prior art often includes terms such as ‘mixing’, ‘high shear mixing,’ ‘rapid mixing’ and the like when describing conditions under which a reaction occurs. These un-quantified parameters often used for mixing efficiency provide little insight into the degree of efficiency to which they are contributing to the overall rate of reaction of the reactants involved.
There is still a need in industry for improved processes and catalysts for hydrogenating fatty acid compositions. The improved catalyst and/or process should reduce or eliminate problems associated with the prior art catalysts and processes. These problems include, but are not limited to, production of products having either an off taste or flavor and/or an unsuitable mouth feel; extended reaction times for hydrogenation that reduce plant throughput; use of expensive catalysts (as is the case with platinum-based catalysts); use of excessive reaction pressures and/or temperatures; production of resulting fatty acids that do not posses the required stability to be used in commercial frying applications; and/or the inability to achieve levels of trans fat and saturated fat that are acceptable to consumers and health experts. Such an improved process for hydrogenation may accelerate the rate of the hydrogenation reaction, for example, by improving the gaseous dissolution of hydrogen in the liquid phase and/or the activity of the catalyst.